In a large number of manufacturing processes utilized in industries such as petrochemicals and food processing, the move toward automation has resulted in the necessity for applying motorized power sources to the components of those manufacturing processes. One of the largest and most critically needed areas of motorized operation is in the opening and closing of valves such as gate valves, butterfly valves, plug valves, and ball valves.
The prior art has provided motorized valve operators which primarily utilize electric motors operating through worm gear assemblies with gear reduction to move the gate valve stem vertically and rotate the valve stems of butterfly and plug valves. The prior art motorized valves have been further automated by the provision of torque sensitive cut-off and reversing switches to shut the actuator motor off before the valve is properly closed. In addition to the automatic operation, a back-up system of manual operation must be provided for the situation where a power loss or motor failure has occurred.
The disadvantages suffered by the prior art valve actuators arise in the structure of the two above-mentioned features, the torque cut-off switch and the manual operation feature. The prior art devices utilize a type of torque sensing system such as a coil spring or Belleville springs interconnected with the worm gear assembly such that when the gate valve is closed and resistance to further movement begins to increase drastically, the spring system is compressed to a point that a switch is actuated. This cuts power to the actuator motor and the motor slows and stops.
The problem that arises with this type of system is that a correct calculation of the closing torque that will be obtained is not possible with any degree of accuracy because of the high rotational inertia in the motor after electric power is shut off. Thus, the only method of getting the proper closing torque is through a trial and error process on each individual motorized operator because of the large number of variables involved, such as motor size, motor speed, total rotational momentum, required seating torque, gear reduction ratio, stem speed, gear masses, and others. As a consequence of this inaccuracy, severe damage can occur to the valve components from inadvertent over-tightening.
A second disadvantage suffered by prior art devices involves the provision for switching the actuator from automatic to manual operation. These devices provide a separate lever and gear assembly in the actuator. This elaborate system begins with an external shifting lever and includes several shafts and sliding gears for shifting the mode of operation. Unfortunately, the mere inertia and complexity of this system may prevent its successful operation when it is needed the most. A small amount of dirt, sludge, or corrosion can render the shifting mechanism inoperable and the valve will not be operable until either the motorized operation is restored or the actuator can be disassembled and repaired. Also, the nature of the manual lever system requires that the actuator be placed in an easily accessible location.
The present invention overcomes these serious disadvantages and other disadvantages of the prior art by providing a motorized valve actuator utilizing electrical and mechanical simultaneous motor disengagement and automatic switching from motor operation to manual operation and back again.